The present invention relates to electrical connectors for insulated electrical conductors, and more specifically relates to crimp connectors of the type comprising a metallic crimp barrel and a heat-shrinkable sleeve.
Crimp connectors are commonly used for establishing an electrical connection between the ends of two electrical conductors. A typical crimp connector comprises a malleable metallic crimp barrel surrounded by a heat-shrinkable sleeve, with a layer of heat-activated adhesive being applied to the inner surface of the sleeve. The bared end portions of two insulated conductors are inserted into the crimp barrel, which is then deformed by a crimping tool to establish an electrical connection between the two conductors. The sleeve is then heated, thereby activating the adhesive and shrinking the sleeve onto the crimp barrel and the conductors to seal the connection. The sleeve of the crimp connector is typically clear to allow visual confirmation that an electrical connection has been made, and is longer than the crimp barrel so as to completely cover the bared end portions of the conductors.
A typical crimp connector is described in U.S. Pat. No. 4,151,364 (Ellis), issued on Apr. 24, 1979. The crimp connector described in this patent comprises an insulating sleeve having a metal crimp barrel permanently positioned therein. This type of crimp connector is manufactured by inserting the crimp barrel into a heat-shrinkable sleeve in its expanded state and then partially shrinking the sleeve down onto the crimp barrel to permanently retain the crimp barrel in the sleeve. Since the sleeve and the crimp barrel are permanently attached, the crimping force must be applied to the crimp barrel through the heat-shrinkable sleeve. It is common to form the heat-shrinkable sleeves of such crimp connectors from a polyolefin which has been crosslinked by electron beam radiation. Sleeves made from this type of material generally have poor resistance to the forces applied by the crimping tool, resulting in splitting of the sleeve to expose the underlying conductors or reduction in the wall thickness of the tube to a point where it is insufficient to provide the necessary physical and dielectric strength.
One solution to this problem is proposed by U.S. Pat. No. 4,196,308 (Siden) issued on Apr. 1, 1980. The Siden patent provides a crimp connector comprising a metal crimp barrel which is removably retained within a heat-shrinkable sleeve. A connection between two conductors is formed by the following steps: the bared end portion of one conductor is inserted into the crimp barrel and the sleeve, the sleeve is slid back from the end of the conductor to expose the crimp barrel, the bared end portion of a second conductor is inserted into the opposite end of the crimp barrel, the exposed crimp barrel is crimped with a crimping tool, the heat-shrinkable sleeve is slid over the connection and is then heated to cause it to shrink over the connection. While the solution proposed by Siden overcomes the problem of splitting or otherwise damaging the heat-shrinkable sleeve, such connectors are more difficult to use since additional steps are required and the user must ensure that the sleeve is properly positioned over the connection prior to heat shrinking. Further, there is the possibility that the crimp barrel and sleeve can become separated and lost prior to use, resulting in further inconvenience.
A number of other solutions have been proposed to make the use of crimp connectors less problematic. One solution involves reduction of the strength of the crimping forces to avoid damage to the sleeve. However, this may result in a crimp connection of unacceptably low quality. Another solution involves shaping the crimping dies of the tool to evenly distribute the crimping forces throughout the wall of the tube. However, such crimping tools are frequently more expensive and consequently less likely to be purchased by a user.
Presently, the most preferred solution for overcoming this problem is to form the heat-shrinkable sleeve from a material which is more resistant to crimping forces than conventional crosslinked polyolefin sleeves. For example, U.S. Pat. No. 4,444,816 (Richards et al.) issued on Apr. 24, 1984, discloses radiation crosslinked polyamides comprising substantial amounts of Nylon-11 and/or Nylon-12 units. These polyamides are heat-shrinkable and are able to withstand the forces applied by a crimping tool without splitting or unacceptable reductions in wall thickness.
However, the use of polyamides to form heat-shrinkable sleeves in crimp connectors is not free from difficulties. These polymers have relatively high softening temperatures, typically about 150xc2x0 C. Heating the electrical connection to these temperatures can damage the insulation of the conductors being joined or may result in excessive melting of the heat activated adhesive, causing it to run out of the connection. There is also the possibility that the user may not sufficiently heat the heat-shrinkable sleeve, resulting in poor sealing of the connection. Furthermore, crimp connectors formed with polyamide sleeves are typically more costly than those made with polyolefin sleeves, and may not have an acceptable degree of clarity which is desired in crimp connectors. Still further, polyamide polymers such as Nylon-11 and Nylon-12 tend to be very rigid, with the result that the metallic conductors may be prone to fatigue failure at the junction with the heat-shrinkable sleeve. It is preferable that the heat-shrinkable sleeve be as flexible as possible in order to provide strain relief to the conductors.
The disadvantages of the prior art discussed above are overcome by the present invention which provides a crimp connector comprising a crimp barrel and a heat-shrinkable sleeve which is formed from an ionic polymer.
Ionic polymers, also known as xe2x80x9cionomersxe2x80x9d, are based on copolymers of xcex1-olefins with ethylenically unsaturated, preferably xcex1,xcex2-ethylenically unsaturated, carboxylic acid monomers in which a proportion of the acid groups of the copolymer are reacted with metal ions to create ionic carboxylates.
One of the earlier patents disclosing ionomers is U.S. Pat. No. 3,264,272 (Rees), issued Aug. 2, 1966. As noted in the Rees patent, ionomers have surprising properties which result from an ionic attraction between the metal ion and one or more ionized carboxylic acid groups. This ionic attraction results in a form of crosslinking which occurs in the solid state. However, when ionomers are heated above their melting point and subjected to shear stresses, the ionic crosslinks are ruptured and the polymers exhibit melt fabricability essentially the same as that of the uncrosslinked linear base copolymer.
It has also been found that ionomer resins have high impact toughness, abrasion resistance and chemical resistance, making them useful in a wide range of consumer and industrial products where these properties are important. Some applications include automobile body parts, bowling pins and cut-resistant golf ball covers. However, ionomers are typically significantly less rigid than polyamide polymers.
The inventors have now found that heat-shrinkable tubing formed from ionomers has high resistance to splitting when subjected to forces of the type applied to a crimp connector by a crimping tool. The resistance to splitting possessed by ionomers is in fact similar to that of presently preferred polyamide heat-shrinkable sleeves.
Thus, the inventors appear to be the first to appreciate that ionomers are suitable for use in heat-shrinkable sleeves of crimp connectors. This is surprising since ionomers are known to be suitable for use in wire coatings (disclosed by Rees) and are known to be suitable for use in heat-shrinkable tubing, as disclosed in U.S. Pat. No. 3,816,335 (Evans) issued Jun. 11, 1974, and in U.S. Pat. No. 5,573,822 (Nishikawa et al) issued Nov. 12, 1996.
The failure of others to appreciate the suitability of ionomers as heat-shrinkable sleeves in crimp connectors is particularly surprising in view of the fact that ionomers are known to possess a number of other properties which are desirable in heat-shrinkable sleeves for crimp connectors, and which render them equally or more suitable to this application than polyamides. In particular, ionomers are known to possess a high degree of transparency; they accept colorants which do not materially deteriorate transparency, allowing for excellent color coding of products; they are typically less expensive than nylon; and can be made with varying degrees of stiffness. Another important advantage of ionomers is that they are heat-shrinkable at significantly lower temperatures than polyamide connector sleeves, typically about 50xc2x0 C. lower. The lower heat shrink temperature renders ionomers more compatible with commonly used heat activated adhesives, such as ethylene-vinyl acetate- (EVA)-based hot melt adhesives, and lessens the likelihood of insufficient heating of the sleeve during heat shrinking and of deterioration of the wire coating due to excessive heating.